1. Field of the Invention
The present invention relates to a lead frame and a manufacturing method therefor, and more particularly to a lead frame and a manufacturing method therefor in which an outer end portion of an inner lead thinner than an outer lead is connected to an inner end portion of the outer lead through an etching stop layer formed of metal different from that of the outer lead.
2. Description of the Related Art
Conventionally known is a lead frame formed by etching a lead frame material having a three-layer structure consisting of a thick copper layer for outer leads, an aluminum layer for etching stops, and a thin copper layer for inner leads (Japanese Patent Application No. Hei 3-274843).
FIG. 1 is a sectional view of such a lead frame in the related art. In FIG. 1, reference numeral 1 generally denotes a lead frame including outer leads of copper, etching stop layers 3 of aluminum formed on inner end portions of the outer leads, respectively, thin inner leads 4 of copper fixed at their outer end portions to the etching stop layers 3, respectively, and bumps 5 of aluminum formed on inner end portions of the inner leads 4, respectively.
The inner end portions of the inner leads 4 are bonded through the bumps 5 to electrode pads 7 of a semiconductor chip 6. The semiconductor chip 6 is sealed by a sealing resin 8.
Such a lead frame 1 is manufactured by individually performing selective etching of the copper layer for the outer leads and selective etching of the copper layer for the inner leads. In the manufacture of the lead frame 1, the etching of the copper layer for the outer leads in performing the selective etching of the copper layer for the inner leads is hindered by the aluminum layer, and the etching of the copper layer for the inner leads in performing the selective etching of the copper layer for the outer leads.
In another technique, the inner leads are formed by plating copper with a resist film used as a mask, so as to avoid that a fine pitch of the inner leads may be hindered by side etching.
Such a conventional lead frame 1 has a problem such that it is difficult to sufficiently increase a bonding strength between the outer leads 2 and the inner leads 4 because there is a limit in increasing a bonding strength between aluminum and copper. That is, the etching stop layers 3 of aluminum are interposed between the outer leads 2 and the inner leads 4 both of copper, and the outer leads 2 are not in direct contact with the inner leads 4.
Further, since the lead frame material having the three-layer structure is used in the manufacture of the lead frame 1, the patterns of the outer leads are necessarily formed by etching one surface of the lead frame material. As a result, an increase in amount of side etching is unavoidable. Accordingly, there is a limit in making the pitch of the outer leads fine, and the demand for very large scale integration cannot be met.
Further, an IC connected to such a lead frame 1 and sealed by the resin 8 is aged in actual use to possibly cause the intrusion of moisture 9 into the resin 8 and therefore generate a local battery by aluminum and copper. As a result, the progress of corrosion of the aluminum due to the moisture 9 is accelerated.
Additionally, in the technique of forming the inner leads by plating copper with a resist film used as a mask, the fine pitch of the inner leads can be expected, but that of the outer leads cannot be expected. The fine pitch of the outer leads has been increasingly demanded by a great increase in degree of integration and number of pins in ICs.